Microtubules are subcellular organelles, comprised primarily of protein subunits termed tubulin, which are found in all higher plants and animals. They perform a variety of functions. These include: directing copies of the genetic material (chromosomes) to daughter cells during growth and development, acting as a cytoplasmic skeleton to maintain the structure of a variety of cells including blood platelets, providing locomotion for sperm cells, involvement in axoplasmic transport, and perhaps granule release by platelets. The presence of microtubules in cells represents a fine balance between the forces which cause their formation and the forces which cause their degradation. Nucleotides such as GTP, which are the building blocks of nucleic acids, stabilize microtubules; other agents such as calcium and the alkaloid, vinblastine, destroy normal microtubule structure. The way in which these agents alter microtubule structure is unclear. Using a variety of fluorescence, kinetic, and chemical techniques, we will perform detailed studies of the interaction of GTP and its analogs with the building block of microtubules, tubulin. These studies will determine which subunit of tubulin is involved in binding, whether binding produces structural alterations, and if so, the nature of these structural alterations. The results of these studies will be correlated with the ability of nucleotides to promote the assembly of Tu. Such studies may shed additional light on how microtubules function in all cells.